X-ray and neutron diffuse scattering from multilayers of fatty acid salt molecules

X-ray (XDS) and neutron (NDS) diffuse scattering are used for nondestructive investigation of the domain structure of organic multilayers prepared by the Langmuir-Blodgett (LB) technique. The multilayers under investigation were lamellae of Cd-stearate and Pb-stearate molecules, XDS measurements are performed to determine the lateral correlation length of scattering aggregates as well as the angular distribution of domains, NDS experiments of sequentially deuterated films show the higher disorder of fatty acid chains compared to the stacking of metallic heads. Holes and scratches are responsible for the density loss within the film and cause the X-rays scattered coherently to vanish completely. We present a simple model to evaluate the XDS and NDS, taking both height fluctuation and density fluctuation of scattering aggregates into account. Height-height correlation given by Sinha and density-density correlation known from small angle X-ray scattering (SAXS) are unified to a general density-density correlation function for small interfaces.